Natural killer cells provide protection from numerous viruses in the early stages of the infection. NK cell activity is controlled by cytokines and cell surface stimulatory and inhibitory receptors. The NKG2D stimulatory receptor has been implicated in the recognition of cells infected with viruses. NKG2D recognizes the RAET1 (including several subfamilies) and MIC families of cell surface ligands, which are poorly expressed on the surface of most normal cells. MIC and RAET1 family members are transcriptionally upregulated in cells infected with herpesviruses and most likely other viruses. Our expertise on immune recognition by NK cells (Raulet) and viral immune evasion strategies (Coscoy) will be combined to investigate our hypothesis that viral infection leads to generic signals that upregulate transcription of NKG2D ligands, but that some viruses evade recognition by preventing ligand expression at the cell surface. The long term aim of these studies is to provide routes to develop therapeutic agents that enhance NK cell responses to viruses, especially in the context of biodefense. Our aims are to 1) focus on two herpesviruses to elucidate mechanisms that upregulate expression of NKG2D ligands in virus-infected cells, including the role of Toll like receptors, cytokines, interferons and the DNA damage pathway;2) Examine representatives of 8 virus families, including several relevant to biofense, for their capacity to upregulate NKG2D ligand mRNAs and downregulate ligand proteins;by examining a panel of mutant fibroblasts defective in a broad array of protective functions, we expect to learn the factors responsible for ligand upregulation in different viral systems;3) Determine how MHV68, a mouse gamma herpesvirus, evades NKG2D ligand upregulation, and, using two-photon microscopy, determine how NKG2D-ligand interactions impact NK cell-target cell interactions in vivo;4) Determine whether the pathways uncovered in the mouse system in aims 1-3 regulate human NKG2D ligands in human cells infected with two herpesviruses, HCMV and KSHV. Importantly, aspects of this project will be part of a collaborative effort between the different groups participating in this PO1 application.